Combinations

## Goal

Combinations are a way to host assets at a different digest path than what their source path compiles to.  Typically this can be used to 'combine' many source files into a single file for the purpose of digesting.  For example:

```ruby
config.combination 'css/base.css', ['css/reset.css', 'css/bootstrap.css']
```

However, combination _could_ be used to alias a single asset:

```ruby
config.combination 'css/web.css', ['css/app.css']
```

Either way, you can refer to both the combination digest path and the individual source digest paths and get the corresponding asset file.  Combination asset files do the obvious: they aggregate the source data for all of their files.

## Implementation

WIP